|
Nathan- In the early 60's I built a beat frequency oscillator metal detector from diagrams and instructions in an electronics magazine. I did it more jest to see if I could do than anything. It was crude, but simple and inexpensive, plus it worked real good. From memory now... I rolled a 1/2" copper tube into a hoop, leaving a gap. Opposite the gap was cut a hole to bring out the search coil wires and run 'em up the handle to the electronics box. A five conductor phone hook up cable was snaked thru the tube. Wires on both ends of the cable were cut staggered about 2", soldered together while exposed in the gap in the tubing and the splices were insulated with tape. Two wires were cut extra long and not spliced. The cable assembly was gradually snaked 1/2 turn inside the tube and the two long ends were pulled out. The hole in the tube and the gap was sealed with tape. That created a five turn search coil, that radiated a magnetic field for ??? around it. The search coil was connected to an oscillator circuit in the electronics box. Inside the box was a second oscillator circuit, that functioned as a reference frequency. The reference frequency was operator knob adjustable and tuned to operate real close to the search coil frequency. A detector circuit operated between the search and reference oscillators. It put out a tone that represented the difference between the two oscillators and fed that signal to an earphone. I'd fire the thing up and let it warm up and the two oscillators stabilize on frequency. Next I'd tune the thing to give a real low frequency (like 10 to 20 cycles per second) buzz. Then head out and sweep the area to be searched. A subtle thing I discovered... By tuning to give an audio signal in the 5 to 25 cps range I could tell the difference between ferros and non-ferros metal! Ferrous would make the audio buzz rate go the opposite direction from non-ferros stuff. Example, if I came over a coin the buzz would slow down, go quiet, and possibly increase as the difference in the two oscillators crossed over. If I came over a pop bottle cap or nail, the buzz would speed up. The buzz rate change was in proportion to the size and closeness of the object. I could read cast iron sewer pipe buried 6' under a lawn. Clay tile wouldn't give a signal. Copper gas line buried at 2' would give a distinct non-ferrous signal.
OK, so you asked the time of day and I described building a watch...
Have fun and find a pot of gold, or that tire eating cultivator shovel! Grins, IHank
| 
